Nozzles, shrouds, sleeves, slide gates and other metal contact parts used in the processing of molten steel, steel superalloys and other specialty metals, and glass are typically manufactured from ceramic materials. Because of its high resistance to erosion from molten steel and the like, purified zirconium dioxide (ZrO.sub.2), zirconia, is a preferred component for fabricating such metal processing articles. Zirconia is typically purified for refractory purposes (to approximately 99% ZrO.sub.2 +HfO.sub.2) from zircon sand using either electrical or thermochemical processes. This material is hereinafter referred to as artificial zirconia to distinguish it from zircon sand, baddeleyite and other naturally occurring forms of zirconia. The electrical process permits the manufacture of large aggregates of purified zirconia which may be crushed to provide a wide range of aggregate piece sizes ranging upwards to several millimeters in diameter desired to achieve good particle packing and high density (i.e. lack of porosity) in the articles formed therefrom. The thermochemical process produces a fine zirconia flour which is typically aggregated by preforming into larger shapes such as dobie type bricks and sintering to high density. The bricks or other shapes are thereafter crushed to produce coarse grain artificial zirconia aggregate. However, the costs involved in refining artificial zirconia from zircon sand and sizing the refined material into appropriate sized aggregate pieces is reflected in the cost of the articles produced.
Baddeleyite is a naturally occurring mineral and by using heavy minerals separation techniques may be concentrated up to 96% or more by weight of zirconium dioxide with traces of hafnium dioxide. Some refractory articles have been made from baddeleyite ore concentrates, but these articles have been limited to fine grained bodies made entirely or almost entirely with materials having effective particle sizes of less than about five microns and, in any event, having no particles greater than about 20 microns. Two exemplary compositions for fabricating such articles are described in U.S. Pat. No. 3,929,498 to Hancock et al. The average particle size of the composition ingredients were less than about 2 microns and of the particles of the sintered aggregate, apart from the aggregation, about 5 to 15 microns. These articles exhibit good resistance to erosion but poor resistance to damage from thermal shock when compared to the performance of comparable articles manufactured from coarse grain (i.e. a millimeter or more in diameter) artificial zirconia.
It has been proposed to use fused baddeleyite concentrates aggregate for fabricating refractory articles. Fused baddeleyite is formed by heating the baddeleyite ore concentrates to a liquid state and then allowing the liquid to cool. It is believed that some form of calcia or other known zirconia stabilizing compositions are typically added to the baddeleyite before melting. The resolidified material is then remilled to provide an assortment of particle sizes including particles several times larger than the largest baddeleyite particles recovered in concentrating the mineral from its ore. While less expensive than artificial zirconia, the steps of melting and subsequently remilling the baddeleyite again add to the cost of the articles produced from the fused material.
It is well known that zirconia in either its naturally occurring or artificial form undergo a 3.8% volume change accompanying a change in phase from a monoclinic to a tetragonal structure at about 1160.degree. C. Phase and structure will be hereinafter referred to interchangeably. To prevent this destructive volume change in dense, very high zirconia content bodies, it is known to stabilize the zirconia by converting it into a cubic phase during sintering, through the addition of zirconia stabilizing compositions. Compositions used for this purpose include yttria (Y.sub.2 O.sub.3), magnesia (MgO) and calcium carbonate which reacts on mild heating to release calcia (CaO) the actual stabilizing agent. The cubic phase formed is stable between room temperature and the melting point of the zirconia (approximately 2700.degree. C.).
Baddeleyite ore concentrates, both fused and unfused, can be distinguished from artificial zirconia prepared by the electrical or thermochemical processes referred to earlier and from other forms of purified zirconia on the basis of uranium 238 and thorium 232 content. Baddeleyite ore concentrates typically contain from about 0.05 to 0.20% by weight uranium 238 and thorium 232 while artificial zirconia contains less than about 0.05% and typically about 0.03% or less uranium 238 and thorium 232.
Additionally, fused baddeleyite may be distinguished from baddeleyite ore concentrates in that gas bubbles and voids are typically formed in the former during the fusing operation. Particles of fused baddeleyite have a "swiss cheese" appearance; that is to say, the particles are characteristically very dense (i.e. non-porous) but with occasional smooth rounded voids or holes. Fused baddeleyite is also often distinguished by microcracks and fissures within the grains from rapid cooling after fusing. These holes, fissures and microcracks are readily ascertainable upon microscopic examination at a magnification of about 50 power and are not apparent in unmelted baddeleyite ore concentrates.